1. Field Of The Invention
The present invention relates to a member of a fiber-reinforced plastic (hereinafter referred to as "FRP") whose internal damage is easily detected by a non-destructive inspection method, and a method of detecting the internal damage of such an FRP member.
2. Description Of The Prior Art
Since FRPs are excellent in mechanical strength with light weight and can easily be molded or formed into desired shapes, they are used for wide applications in various fields. Despite such excellent characteristics, the FRPs suffer from problems such as micro-cracking, the separation of reinforcing fibers from matrix plastics, etc., which may be caused by impact from outside or fatigue after the lapse of a long period of time. Such defects would lead to the failure of the FRPs.
It is, therefore, important to know a limit until which the FRPs can be used safely. For this purpose, various methods of detecting defects in the FRPs have been proposed so far.
For example, the detection of the extent of internal damage in the FRPs using a light transmission method, an X-ray method, an ultrasonic method or an acoustic emission (AE) method has been developed. However, the light transmission method, the X-ray method and the ultrasonic method are suitable only for detecting relatively large defects, and the AE method requires some troublesome operations such as the application of a load to the FRP member, though it can detect relatively small defects. Further, the AE method gives unsatisfactory precision in the detection of the defects.
As an alternative method, Japanese Patent Laid-Open No. 60-114741 discloses a method of detecting internal damage of FRP members, in which long, conductive carbon fibers are embedded in the FRP member, and electric current is supplied to the long, conductive carbon fibers to measure the electric conductivity of the carbon fibers, by a level of which electric conductivity the extent of internal damage of FRP members can be predicted. This method is based on the principle that if there is an internal damage in the FRP member, some of the carbon fibers would be cut in the damaged portion, resulting in no or reduced electric conductivity of the carbon fibers.
However, since this method requires troublesome and difficult steps of attaching long carbon fibers to surfaces of the reinforcing fiber cloths, this method is not suitable for the mass production of the FRP members. Also, since the long carbon fibers are used, this method is not suitable for FRP members with complicated shapes. Further, this method may accelerate the deterioration of the FRP members because electric current is applied to the conductive carbon fibers contained in the FRP members.
Therefore, a simple detection method of internal damage in FRP members in a non-destructive manner has been desired.